The present invention relates to a method and an apparatus for multiplexing, for example, a digital audio signal with a digital video signal by time division and transmitting and/or receiving such multiplexed signals as serial data, and more particularly, to a method and an apparatus capable of transmitting and/or receiving multiplexed signals even in a state where an audio signal is not synchronized with a video signal.
For transmission of a digital video signal, there is known an exemplary system employing a serial digital interface. According to this interface, a digital video signal is transmitted as 1-bit serial data. More specifically, there is proposed in the serial digital interface a technique in which a blanking period of the video signal is rendered vacant due to non-transmission of horizontal and vertical sync signals, and a digital audio signal is transmitted during such blanking period through time-base compression and multiplexing.
In U.S. Pat. No. 5,199,030, there is disclosed a method of multiplexing and transmitting a digital video signal and a digital audio signal.
The serial digital interface mentioned above has the merit that both a digital video signal and a digital audio signal can be transmitted via a single coaxial cable without signal deterioration, whereby the signal connection between various digital video apparatus can be remarkably simplified.
Generally, the sampling clock frequencies for the digital video signal and the digital audio signal are different from each other. For example, the sampling rate in the case of NTSC video signal is 14.3 MHz, whereas the rate for the audio signal is 48 kHz. The transmission clock rate for the audio signal is generally set to be equal to that for the digital video signal. Assuming in the above example that each sample is composed of 10 bits, it follows that the transmission rate is 143 Mbps. The audio data is compressed in its time base and is multiplexed to form data of the above transmission rate.
In this case, for reproducing the digital audio data, it is generally necessary that the sampling clock on the transmission side and that on the reception side are coincident with each other. However, an occurrence of a frequency error is practically unavoidable even by the use of quartz oscillators. For attaining synchronism between the clocks on the transmission and reception sides, it is customary in the prior art to adopt means of synchronizing, on the transmission side, the sampling clock of the audio data with the video signal while using, on the reception side, the vertical sync signal included in the received video signal, and locking the clock frequency of the video signal to the reproduced sync signal in the same manner as on the transmission side, thereby achieving satisfactory reproduction of the audio data.
When a digital audio signal is transmitted via a serial interface while being multiplexed by time division with a digital video signal as described above, the following three requisite relations need to be ensured.
(1) The transmission audio clock is synchronized with the transmitted video signal. PA1 (2) The transmission video signal and the received video signal are synchronized with each other. PA1 (3) The reception audio clock is synchronized with the received video signal. Due to the above requirements, there exists the following implicit precondition. PA1 (4) The transmission audio clock and the reception audio clock are synchronized with each other.
If the conditions (1) to (4) described above fail to be satisfied, there arises some excess or deficiency of the data on the reception side due to the frequency difference between the transmission audio clock and the reception audio clock, whereby the original digital audio signal is not reproduced properly.
Therefore, in the conventional transmission system employing such a digital interface, it is impossible to transmit an audio signal not synchronized with a video signal.
However, in a practical operation, it is not exactly ensured that a desired relationship of synchronism is maintained between the digital video signal and the digital audio signal. For example, though in any broadcasting station a video signal and an audio signal are locked to sync signals of that station, such sync signals of that station may not always be synchronized with the signals of the other station, so that the above problem is raised in using a television signal of the other station.
Although it is possible to attain synchronism with respect to the video signal in this case by locking the same to a sync signal, there still remains a problem that the clock for the audio signal may fail to follow up exactly.
Further in any program employing a VTR, there may be selected, depending on the content of a broadcast, a program play mode where the broadcast is so adjusted as to be completed within a changed time shorter or longer than a normal reproduction time. Supposing now that the time is shortened by 5 percent for example, the audio clock frequency is also shifted to be higher by 5 percent to consequently cause a non-locked state.